The fundamental work of Donaldson and Uhlenbeck-Yau proves the the smooth convergence of the Yang-Mills flow of stable integrable unitary connections on hermitian vector bundles over Kaehler manifolds. This was generalized by Bando and Siu to incorporate certain (singular) hermitian structures on reflexive sheaves. Bando-Siu also conjectured what happens when the initial sheaf is unstable; namely, that the limiting behavior should be controlled by the Harder-Narasimhan filtration of the sheaf. In this talk I will describe the solution to this question, which draws on the work of several authors.